Due to the decreasing supply of copper conventionally employed in automotive heat exchangers, aluminum has become important as an alternative material source. However, in a number of processes used in fabricating aluminum parts, such as brazing required in an automotive heat exchanger, the stability of the surface oxide film, present on aluminum, is a definite impediment. In brazing, the film acts as a barrier to wetting and flow of the filler metal required for joint formation. Oxide removal and prevention of reoxidation are the principal requirements for a successful aluminum joining method.
Fluxless or vacuum brazing has assumed a position of commercial importance because it does not require the removal of a flux residue and reduces the susceptibility to aqueous corrosion of parts having this flux residue. Vacuum or fluxless brazing essentially comprises placing the assembled structure to be brazed into a heating furnace with a filler metal disposed at the sites to be joined. The furnace is evacuated to a vacuum level of about 10.sup.-4 -10.sup.-5 Torr and simultaneously heated to a temperature in excess of the melting point of the filler metal, but below the melting point of the base metal to be joined. It has been conventional to employ a furnace which heats at a rate of approximately 15.degree. C./min so that the entire brazing operation is accomplished within a period of about 15 to 18 min.
Although in most instances, vacuum brazing produces a sound joint, there are instances when certain vapor phase constituents within the furnace atmosphere degrade braze performance. Degradation occurs during the stage when the promoter reacts with the oxide film and later when capillary forces tend to draw at least part of the fluidized filler metal through the oxide film. In spite of the presence of promoter agents to facilitate wetting of the oxide film, the presence of such vapor phase constituents counteract the beneficial effect of the promoter agents.